Materials currently used in RF front end modules are as follows:
1. SOQ (silicon on quartz), SOS (silicon on sapphire): SOQ is as same as traditional SOI (silicon-on-insulator), which generate a lower leakage current, since its lower parasitic capacitance, the circuit performance under high frequency is enhanced. The advantage of SOS is its extremely good electrical insulation, which can effectively prevent radiation caused by stray currents from spreading to nearby elements. The substrates like SOQ and SOS can get excellent RF performance, but this kind of structure is very rare, so they are very expensive.
2. High-resistivity silicon substrates: the resistivity of it is more than 500 ohm·cm, this kind of substrate is worse than the first, this kind of substrate is not benefiting from the structure advantages of SOI type, but they cost less.
3. High-resistivity SOI substrate: this kind of substrate has structural advantages, but the performance is worse than the first.
A reason of the formation of low-resistivity layer is: due to the existing of pollutants on the surface of the low-resistivity layer before bonding, during the process of bonding, these pollutants is packaged in the bond interface and can diffuse to the high-resistivity substrate; another reason of the formation of low-resistivity layer is: the high content of oxygen atoms in the substrate, so it's necessary to perform a heat treatment, to precipitate the oxygen atoms to get the high-resistivity substrate. However, the diffusion of oxygen atoms and the process of heat treatment lead to the low resistivity of the surface of the formed substrate. It is difficult to control the two reasons now.
4. Based on the 3, the high-resistivity SOI substrate is improved by adding defect layer: to achieve the purpose, lots of techniques are tried, but there are some disadvantages: sensitive to SOI manufacturing and generating heat in the subsequent process of IC components fabrication, not easy to make materials with good thermal stability.